Is it possible to composite a scene without rendering from the command line?
Yes, there is a better way to do this. If your composting process doesn't require a render, it should be in its own file. You would do whatever rendering you need, save those out to OpenEXR (or some other format) and use an Image Node to read in your image, or sequence of images and run those through your compositing process.

The way that you "skip" rendering is by deleting all "Render Layer" input nodes and all 3D cameras. This causes problems if you need cameras in your scene, which is why your composting should be in another Blender project.

Alternately, you could put the compositing in another scene in the same Blender file, if that's how you're organizing things.